1. Field of the Invention
This invention relates to a method of making a hologram, and in particular to a method of making a hologram imaged substantially without aberrations during use when the wavelength of light in making the hologram differs from the wavelength of light during the use and having high diffraction efficiency.
2. Description of the Prior Art
A hologram lens can be obtained by making a hologram of a point source of light by the use of the holography technique. The hologram lens is of planar shape and has advantages that it is a thin film lens having a thickness of the order of several microns and that a number of lenses can be mass-produced on the same flat plate by the step-and-repeat method. Therefore, the utilization of the hologram lens as an optical element in an optical system utilizing a laser light, such as the condensing lens of the optical head of an optical disc device or a collimation lens for converting the divergent light beam from a semiconductor laser into a parallel light beam, has been proposed.
The optical system of the optical head portion of the optical disc device is such that a condensing hologram lens is disposed on the front surface side of a disc substrate so as to read through the disc substrate a signal recorded on the back surface of a plastic plate usually of a thickness of the order of 1.1 mm which is the disc substrate. The hologram lens is disposed with an air space of the order of 1 mm with respect to the disc substrate so that no collision is caused by vibration of the disc, and a cover glass or protective layer of a suitable thickness is interposed therebetween to prevent adherence of dust or the like to the hologram lens.
An example of the optical system for making the hologram lens used in such an optical system is shown in FIG. 1 of the accompanying drawings. In FIG. 1, part of a monochromatic light 2 emitted from a laser light source 1 is transmitted through a half-mirror 3, is reflected by a reflecting mirror 4 and is condensed in a pin-hole 16 by a microscope objective lens 15, and the light transmitted through the pin-hole 16 is transmitted through a collimation lens 17 and becomes a parallel light beam 18, which is transmitted through a parallel flat plate 9 and enters a hologram sensitive material 11 applied to a hologram substrate 10. This is a reference light. On the other hand, the light beam reflected by the half-mirror 3 is reflected by a reflecting mirror and is condensed in a pin-hole 8 by a microscope objective lens 7, and the light transmitted through the pin-hole 8 becomes a divergent light beam 12, which is transmitted through the parallel flat plate 9 and enters the hologram sensitive material 11. This is an object light. The object light beam 12 is made into a divergent light beam having spherical aberration by the parallel flat plate 9, and this light beam and the reference light beam form interference fringes at the position of the hologram sensitive material 11, and these interference fringes are recorded on the hologram sensitive material 11. By developing this, there is obtained a hologram lens.
Where the hologram lens thus made is used, a laser light of the same wavelength as that of the laser light used during the forming of the hologram lens is made into a parallel light beam at the same angle as the parallel light beam 18 but in the reverse direction and is caused to enter the hologram 11. The light diffracted by the hologram 11 becomes a convergent light beam having the spherical aberration imparted to the object light during the making and, after this has been transmitted through the cover glass and the disc substrate, a light spot is produced at a position corresponding to the pin-hole 8 during the making of the hologram.
Thus, by using light of the same wavelength during the making and during the use, complete wave surface reproduction can be accomplished by the hologram lens substantially without aberrations.
Particularly, where a volume type phase hologram is made with bichromate gelatine or the like used as the hologram sensitive material 11, the diffraction efficiency of the hologram can be improved up to approximately 100% and the utilization efficiency of light becomes sufficient.
Presently, it is preferable that a compact, light-weight semiconductor laser requiring no special modulator be used as the light source in an optical system using a hologram. The oscillation wavelength range of such a semiconductor laser is usually from the near-infrared range to the infrared range (0.78 .mu.m or more). Accordingly, where the making of the hologram lens as described above and the image reproduction using the same are to be effected by the use of such semiconductor laser, it is necessary to use a hologram sensitive material having effective sensitivity at 0.78 .mu.m or more. As a hologram sensitive material having sensitivity in this wavelength range, there is a silver salt sensitive material sensitized by infrared light. However, the hologram made by the use of this sensitive material is an absorption type hologram and therefore has the disadvantage that its diffraction efficiency is as low as several %. By means of bleaching, the diffraction efficiency may be improved to a certain degree, but there is a limit to this.
Accordingly, to improve the diffraction efficiency, it is necessary to adopt a volume type phase hologram, and the aforementioned bichromate gelatine is typical as a sensitive material used for making of such a hologram, but in this sensitive material, the effective sensitivity area is up to green light having a maximum wavelength of 0.55 .mu.m, and it is merely possible to endow such sensitive material with the sensitivity up to red light of 0.6 .mu.m even if special pigment sensitization is effected thereon. Further, a volume type hologram sensitive material having effective sensitivity in the near-infrared range and the infrared range is not yet known.
Therefore, no semiconductor laser can be used during the making of a volume type phase hologram, but a laser of a shorter wavelength is used. When the hologram made in this manner is used in an optical system using a semiconductor laser, the wavelength of light differs during the making and during the use and therefore, it is not imaged without aberrations and thus, in some cases, aberration correction becomes necessary.
Further, in the hologram making optical system as shown in FIG. 1, the parallel flat plate 9 is disposed immediately forwardly of the hologram sensitive material 11, and this results in creation of harmful ghost images. That is, a light beam 13 resulting from part of the object wave light beam 12 being reflected by a second surface and then a first surface of the parallel flat plate 9 or a light beam 13' resulting from part of the object wave light beam 12 being reflected by the surface of the hologram sensitive material 11 and then the second surface of the parallel flat plate 9 enters the hologram sensitive material 11, whereby harmful ghost images are recorded. These ghost images are reproduced during the use of the hologram lens and may thus result in creation of unnecessary ghost light and reduction in diffraction efficiency.
For the reasons set forth above, it has been difficult in the conventional making method to make a hologram used in a light of a wavelength different from the wavelength of the light during the making. However, there is a method in which, as shown, for example, in Japanese Laid-open Patent Application No. 74708/1982, a suitable aberration is pre-imparted to a hologram making light beam and a hologram is made so that there is no aberration when it is used in a different wavelength, but in this case, the optical system for imparting a suitable aberration is not coaxial and use has been made of such means as inclining the optical element. Accordingly, the designing of such optical system for making a hologram has been very cumbersome and has required much labor and, when it is to be actually disposed, strict positional accuracy has been required and therefore, the setting thereof has also required much labor.